This invention relates to a combined electrical and fluid operated control system for closing a fluid flowline in the event of unusual pressure conditions therein.
In the past, fluid operated safety systems have been used to shut-in oil and gas wells when the pressure in the production flowline of the well is unduly high or low. The safety system typically includes a gate valve on the wellhead and pneumatic pressure sensors on the flowline which fire in a manner to effect closing of the gate valve if the fluid pressure in the production line is outside of the operating range of the system. Although fluid systems of this type have functioned in a generally satisfactory manner, they have not been wholly free of problems. Perhaps most importantly, such systems require a number of pressure containing components which are susceptible to a variety of mechanical problems such as seal failure and the like. Also, long fluid lines are sometimes required and the cost and maintenance problems of the safety system are increased accordingly. Another problem has been to assure that the system is automatically locked out of service so that it must be manually reset once it has fired to shut-in the well. A lockout feature of this nature enhances the safety of the system in that it assures that the well will be shut-in until the defect is corrected and the safety system is returned to service by positively resetting it.
Along with the surface safety valve on the wellhead, it is common for producing wells to also have a subsurface safety valve which is typically a ball valve located down in the well bore a considerable distance below the surface. For economic reasons, it is important to protect the subsurface valve because of the high costs involved in repairing or replacing it, particularly when the well is in an inaccessible location such as in a subsea enviroment. Accordingly, for protection of the subsurface valve, it is desirable for the safety system to close the subsurface valve only after the surface valve has already been closed and to open the subsurface valve prior to opening of the surface safety valve.